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SAN FRANCISCO — The self-driving car is edging closer to becoming driverless.

Waymo, the autonomous-car company from Google’s parent company Alphabet, has started testing a fleet of self-driving vehicles without any backup drivers on public roads, its chief executive officer said Tuesday. The tests, which will include passengers within the next few months, mark an important milestone that brings autonomous-vehicle technology closer to operating without any human intervention.

Dozens of companies are testing self-driving technology on public roads across the United States and some autonomous features are available in today’s cars. But Waymo is believed to be the first company to test vehicles on public roads without a driver ready to take over in an emergency.

“Our ultimate goal is to bring our fully self-driving technology to more cities in the U.S. and around the world,” John Krafcik, Waymo’s chief, said in prepared remarks at a technology conference in Portugal on Tuesday. “Fully self-driving cars are here.”

The tests are a show of engineering prowess by Waymo at a time when traditional automakers and other tech companies like Uber race to develop similar vehicles.

Waymo is limiting the trials to a region around Phoenix, where it has been conducting a ride-testing program this year, and plans to expand the testing area over time. The company said it planned to use the driverless vehicles to launch a commercial ride-hailing service for the general public, but did not offer any detail on when, where or how.

Waymo said its driverless cars hit public roads last month. The company did not say whether it was testing the driverless cars in environments considered challenging for autonomous vehicles, like bridges or tunnels, or more difficult conditions, like driving at night or in rain and snow — usually not a big concern in the dry Phoenix climate.

While the prospect of cars without emergency drivers may raise concerns among some passengers, Waymo said it had confidence in the safety of its self-driving technology. It has included backup systems like a secondary computer to take over if the main computer fails. And though the cars are driverless, they are not entirely without humans, at least for now. Waymo employees sit in the back seat of the cars, monitoring them, a company spokesman, Johnny Luu, said.

Once passengers join the tests, they will be able to contact Waymo support staff with a button inside the car. If the cars are involved in a crash, they are programmed to respond appropriately, including pulling off the road on their own.

Driverless cars are regulated by a patchwork of state laws. Arizona, like many states, has no restrictions against operating an autonomous vehicle without a person in the driver’s seat. On the other hand, California, where Waymo is headquartered, requires any self-driving car to have a safety driver sitting in the front.

In December, Waymo published a report for California’s Department of Motor Vehicles about how frequently its car “disengaged” — deactivating its autonomous mode because of a system failure or safety risk and forcing a human driver to take over. In the report, Waymo said this happened once every 5,000 miles the cars drove in 2016, compared with once every 1,250 miles in 2015.

Consumer Watchdog, a frequent critic of Alphabet, said that data demonstrated that the cars are not ready to drive without any human intervention and that Waymo was following the Silicon Valley model of “beta testing” a new technology on the public.

“It’s the wrong approach when you’re dealing with self-driving cars,” said John M. Simpson, a director at Consumer Watchdog. “When things go wrong with a robot car, you kill people.”

Researchers believe self-driving cars can be safer than cars operated by human drivers because they are programmed to adhere strictly to traffic laws, they don’t get distracted, and they usually refrain from taking unnecessary risks.

Waymo, which started as a research and development project for Google in 2009, maintains what many in the industry consider a technological advantage over its competitors. Waymo said its autonomous vehicles have driven more than 3.4 million miles on actual roads — with safety drivers — as well as running 10 million miles every day in a virtual simulator.